Description of problem:
iface-start/destroy should add checkpoint of interface's status before execute, or that will cause iface-start/destroy an interface which defined with static IP(contain ipv4 and ipv6) always return 0(successfully) even the interface is already in active/inactive status.

Version-Release number of selected component (if applicable):
# rpm -qa libvirt qemu-kvm; uname -r
qemu-kvm-1.4.0-2.1.el7.x86_64
libvirt-1.0.4-1.1.el7.x86_64
3.9.0-0.rc7.52.el7.x86_64

How reproducible:
100%

Steps:
1. Stop the NetworkManager service.
Prepare a interface xml(replace MAC addr to your own one):
# cat net-eth2-ipv4.xml
<interface type='ethernet' name='eth2'>
  <start mode='onboot'/>
  <mac address='00:1b:21:39:8b:19'/>
  <protocol family='ipv4'>
    <ip address='10.66.6.91' prefix='23'/>
  </protocol>
</interface>

2. Undefine the pre-existing interface eth2(using iface-destroy, and iface-undefine)

3. Define new eth2 with static IP

# virsh iface-define net-eth2-ipv4.xml
Interface eth2 defined from net-eth2-ipv4.xml

# virsh iface-start eth2
Interface eth2 started

# ifconfig eth2
eth2: flags=4163<UP,BROADCAST,RUNNING,MULTICAST>  mtu 1500
        inet 10.66.6.91  netmask 255.255.254.0  broadcast 10.66.7.255
        inet6 fe80::21b:21ff:fe39:8b19  prefixlen 64  scopeid 0x20<link>
        ether 00:1b:21:39:8b:19  txqueuelen 1000  (Ethernet)
        RX packets 130589  bytes 10167550 (9.6 MiB)
        RX errors 0  dropped 25  overruns 0  frame 0
        TX packets 5662  bytes 1257190 (1.1 MiB)
        TX errors 0  dropped 0 overruns 0  carrier 0  collisions 0
        device memory 0xf4820000-f4840000  

# virsh iface-list --all
Name                 State      MAC Address
--------------------------------------------
eth0                 active     d83:85:7e:61:9b
eth1                 active     00:1b:21:39:8b:18
eth2                 active     00:1b:21:39:8b:19
lo                   active     00:00:00:00:00:00

4. do not destroy eth2, re-start it:
# virsh iface-start eth2
Interface eth2 started

# echo $?
0

# virsh iface-start eth2
Interface eth2 started

# echo $?
0

# virsh iface-start eth2
Interface eth2 started

# echo $?
0

Always start successfully even the interface is in active status.

Actual results:
As steps

Expected results:
Iface-start should fail when interface is already in active status.

Additional info:

If interface is in active status,
# ifup eth2
RTNETLINK answers: File exists
will get some suitable reply for the user, but iface-start just masked it and return nothing.

For destroy problem, they(iface-destroy and ifdown) are working as the same operation.

IMO, if we want more suitable custom taste for this feature, we'd better add this checkpoint for it.


Even more, iface-define also need do some checkpoint for existing interface to modify the ifcfg-* file careless:
# virsh iface-list --all
Name                 State      MAC Address
--------------------------------------------
eth0                 active     d83:85:7e:61:9b
eth1                 active     00:1b:21:39:8b:18
lo                   active     00:00:00:00:00:00

define a static IP interface named eth0 will also successfully without any prompt.
# cat net-static-eth0.xml
<interface type='ethernet' name='eth0'>
  <start mode='onboot'/>
  <protocol family='ipv4'>
    <ip address='10.66.4.54' prefix='23'/>
  </protocol>
  <protocol family='ipv6'>
    <ip address='3ffe::200' prefix='64'/>
  </protocol>
</interface>

# virsh iface-define net-static-eth0.xml
Interface eth0 defined from net-static-eth0.xml

That will change ifcfg-eth0 file:
# cat /etc/sysconfig/network-scripts/ifcfg-eth0
DEVICE=eth0
ONBOOT=yes
BOOTPROTO=none
IPADDR=10.66.4.54
NETMASK=255.255.254.0
IPV6INIT=yes
IPV6_AUTOCONF=no
DHCPV6=no
IPV6ADDR=3ffe::200/64

And start eth0 again:
# virsh iface-start eth0
Interface eth0 started

that will cause iface-dumpxml get un-coincident xml info with ifconfig eth0:
# virsh iface-dumpxml eth0
<interface type='ethernet' name='eth0'>
  <mac address='d83:85:7e:61:9b'/>
  <protocol family='ipv4'>
    <ip address='10.66.5.8' prefix='22'/>
    <ip address='10.66.4.54' prefix='23'/>    --- will cause the interface have two ip addr
  </protocol>
  <protocol family='ipv6'>
    <ip address='3ffe::200' prefix='64'/>     --- and also for ipv6
    <ip address='fe80:ad3:85ff:fe7e:619b' prefix='64'/>
  </protocol>
</interface>

# ifconfig eth0
eth0: flags=4163<UP,BROADCAST,RUNNING,MULTICAST>  mtu 1500
        inet 10.66.5.8  netmask 255.255.252.0  broadcast 10.66.7.255
        inet6 3ffe::200  prefixlen 64  scopeid 0x0<global>
        inet6 fe80:ad3:85ff:fe7e:619b  prefixlen 64  scopeid 0x20<link>
        ether d83:85:7e:61:9b  txqueuelen 1000  (Ethernet)
        RX packets 94448  bytes 8385061 (7.9 MiB)
        RX errors 0  dropped 0  overruns 0  frame 0
        TX packets 2192  bytes 632688 (617.8 KiB)
        TX errors 0  dropped 0 overruns 0  carrier 0  collisions 0
        device interrupt 17