Description of problem:
I have a kvm guest (named i386_kvm) with 2 serial ports (ttyS0, ttyS1). Plymouth is interfering with the latter, making it impossible to log in via "virsh console". 

1. KVM guest
-------------

Relevant part of guest XML:
  <serial type="file">
    <source path="/mnt/tests/i386_kvm_console.log"/>
    <target port="0"/>
  </serial>
  <serial type="pty">
    <target port="1"/>
  </serial><console type="file">
    <source path="/mnt/tests/i386_kvm_console.log"/>
    <target type="serial" port="0"/>
  </console>

i386_kvm # cat /proc/cmdline
ro root=/dev/mapper/vg_dhcp4798-lv_root console=ttyS0,115200 console=ttyS1,115200 rd_NO_LUKS LANG=en_US.UTF-8 rd_LVM_LV=vg_dhcp4798/lv_swap rd_LVM_LV=vg_dhcp4798/lv_root rd_NO_MD SYSFONT=latarcyrheb-sun16 console=ttyS0   KEYBOARDTYPE=pc KEYTABLE=us rd_NO_DM

i386_kvm # cat /etc/init/ttyS1.conf
# start ttyS1
start on runlevel [2345]
stop on runlevel [S016]
instance ttyS1
respawn
pre-start exec /sbin/securetty ttyS1
exec /sbin/agetty /dev/ttyS1 115200 vt100-nav

Plymouth has ttyS1 opened and is activately reading from it, which is presumably the reason why the whole input is not reaching getty:

i386_kvm # ps afx | grep plymouth
  110 ?        S      0:00 /bin/plymouthd --attach-to-session
 1884 pts/1    S+     0:00          \_ grep plymouth
 
i386_kvm # ls -la /proc/110/fd
total 0
dr-x------. 2 root root  0 Dec 15 08:31 .
dr-xr-xr-x. 8 root root  0 Dec 15 03:15 ..
lrwx------. 1 root root 64 Dec 15 08:31 0 -> /dev/tty1
lrwx------. 1 root root 64 Dec 15 08:31 1 -> /dev/tty1
lrwx------. 1 root root 64 Dec 15 08:31 10 -> /dev/ttyS0
lrwx------. 1 root root 64 Dec 15 08:31 11 -> /dev/ttyS0
l-wx------. 1 root root 64 Dec 15 08:31 12 -> /var/log/boot.log
lrwx------. 1 root root 64 Dec 15 08:31 2 -> /dev/tty1
lrwx------. 1 root root 64 Dec 15 08:31 3 -> [eventpoll]
lr-x------. 1 root root 64 Dec 15 08:31 4 -> pipe:[5891]
l-wx------. 1 root root 64 Dec 15 08:31 5 -> pipe:[5891]
lrwx------. 1 root root 64 Dec 15 08:31 6 -> /dev/ptmx
lrwx------. 1 root root 64 Dec 15 08:31 8 -> socket:[5896]
lrwx------. 1 root root 64 Dec 15 08:31 9 -> /dev/ttyS1

i386_kvm # strace -f -t -p 110
Process 110 attached - interrupt to quit
08:33:15 clock_gettime(CLOCK_MONOTONIC, {1096, 378922781}) = 0
08:33:15 epoll_wait(3, {{EPOLLIN, {u32=134944952, u64=134944952}}}, 4, -1) = 1
08:33:41 clock_gettime(CLOCK_MONOTONIC, {1122, 159190199}) = 0
08:33:41 poll([{fd=9, events=POLLIN|POLLPRI}], 1, 10) = 1 ([{fd=9, revents=POLLIN}])
08:33:41 read(9, "o", 1024)             = 1
08:33:42 epoll_wait(3, {{EPOLLIN, {u32=134944952, u64=134944952}}}, 4, -1) = 1
08:33:42 clock_gettime(CLOCK_MONOTONIC, {1123, 569203553}) = 0
08:33:42 poll([{fd=9, events=POLLIN|POLLPRI}], 1, 10) = 1 ([{fd=9, revents=POLLIN}])
08:33:42 read(9, "t", 1024)             = 1
08:33:43 epoll_wait(3, {{EPOLLIN, {u32=134944952, u64=134944952}}}, 4, -1) = 1
08:33:44 clock_gettime(CLOCK_MONOTONIC, {1125, 443173055}) = 0
08:33:44 poll([{fd=9, events=POLLIN|POLLPRI}], 1, 10) = 1 ([{fd=9, revents=POLLIN}])
08:33:44 read(9, "", 1024)              = 0
08:33:44 epoll_wait(3, {{EPOLLIN|EPOLLERR|EPOLLHUP, {u32=134944952, u64=134944952}}}, 4, -1) = 1
08:33:44 clock_gettime(CLOCK_MONOTONIC, {1125, 454963448}) = 0
08:33:44 ioctl(9, FIONREAD, [0])        = -1 EIO (Input/output error)
08:33:44 open("/dev/ttyS1", O_RDWR|O_NOCTTY) = 7
08:33:44 epoll_ctl(3, EPOLL_CTL_ADD, 7, {EPOLLERR|EPOLLHUP, {u32=134951872, u64=134951872}}) = 0
08:33:44 epoll_ctl(3, EPOLL_CTL_MOD, 7, {EPOLLERR|EPOLLHUP, {u32=134951872, u64=134951872}}) = 0
08:33:44 fstat64(7, {st_mode=S_IFCHR|0620, st_rdev=makedev(4, 65), ...}) = 0
08:33:44 ioctl(7, SNDCTL_TMR_TIMEBASE or TCGETS, {B9600 opost -isig -icanon -echo ...}) = 0
08:33:44 ioctl(7, SNDCTL_TMR_START or TCSETS, {B9600 opost -isig -icanon -echo ...}) = 0
08:33:44 ioctl(7, SNDCTL_TMR_TIMEBASE or TCGETS, {B9600 opost -isig -icanon -echo ...}) = 0
08:33:44 epoll_ctl(3, EPOLL_CTL_MOD, 7, {EPOLLIN|EPOLLERR|EPOLLHUP, {u32=134951872, u64=134951872}}) = 0
08:33:44 epoll_ctl(3, EPOLL_CTL_DEL, 9, NULL) = 0
08:33:44 epoll_wait(3, ^C <unfinished ...>
Process 110 detached

2. Host
--------
host # virsh console --devname serial1 i386_kvm
Connected to domain i386_kvm
Escape character is ^]

Red Hat Enterprise Linux Server release 6.5 (Santiago)
Kernel 2.6.32-435.el6.i686 on an i686

dhcp47-98 login: ro
Password:

PROBLEM
--------
"root" was entered, but only "ro" reached getty.

Workaround
-----------
Moving plymouth away makes the issue go away and I can log in to guest.

i386_kvm # mv /sbin/plymouthd /sbin/disable_plymouthd
i386_kvm # dracut -f
i386_kvm # reboot

host # virsh console --devname serial1 i386_kvm
Connected to domain i386_kvm
Escape character is ^]
...
dhcp47 login: root
Password:

# stty
speed 115200 baud; line = 0;
-brkint ixoff -imaxbel
-iexten -echoctl

Version-Release number of selected component (if applicable):
plymouth-0.8.3-27.el6.x86_64

How reproducible:
100%

Steps to Reproduce:
1. install kvm guest with 2 serial ports
2. open virsh console to ttyS1
3. try to log in

Actual results:
unable to log in, input is getting lost

Expected results:
login works on ttyS1

Additional info: