Description of problem:
Used 'wavcapture' command to capture audio to a wave file, but the file is 0 bytes, does it the expected result? It can be hit both in rhel6 and rhel7 host. I checked the bug 749830 #c15 and #16 comments that not zero bytes. 

Version-Release number of selected component (if applicable):
host info:
kernel-3.9.0-0.55.el7.x86_64
qemu-kvm-1.4.0-4.el7.x86_64
guest info:
kernel-3.9.0-0.55.el7.x86_64

How reproducible:
100%

Steps to Reproduce:
1.launch a guest by.
# /usr/libexec/qemu-kvm -M pc-i440fx-1.4 -enable-kvm -m 2048 -smp 2,sockets=1,cores=2,threads=1 est -uuid `uuidgen` -rtc base=utc,clock=host,driftfix=slew -boot menu=on -drive file=/home/RHEL-Server-7.0-64-virtio.raw,if=none,id=drive-virtio-0-0,media=disk,format=raw,cache=none,werror=stop,rerror=stop -device virtio-blk-pci,drive=drive-virtio-0-0,id=virt0-0-0 -net none -usb -device usb-tablet,id=input1 -spice port=9000,disable-ticketing -vga qxl -global qxl-vga.vram_size=67108864 -monitor stdio -balloon none
2.Used 'wavcapture' command to capture audio to a wave file.
(qemu) wavcapture foo 44100 7
incorrect bit count 7, must be 8 or 16
Failed to add wave capture
(qemu) info capture 
(qemu) wavcapture test
(qemu) info capture 
[0]: Capturing audio(44100,16,2) to test: 0 bytes
(qemu) wavcapture test
(qemu) info capture 
[0]: Capturing audio(44100,16,2) to test: 0 bytes
[1]: Capturing audio(44100,16,2) to test: 0 bytes

Actual results:
Used 'wavcapture' command to capture audio to a wave file, but the file is 0 bytes.

Expected results:
it should not always 0 bytes, iy should have size.

Additional info:
I checked the related code from /root/qemu/audio/wavcapture.c
...
static void wav_capture (void *opaque, void *buf, int size)
{    
    WAVState *wav = opaque;
     
    if (fwrite (buf, size, 1, wav->f) != 1) {
        monitor_printf (cur_mon, "wav_capture: fwrite error\nReason: %s",
                        strerror (errno));
    }
    wav->bytes += size;
}
...
static void wav_capture_info (void *opaque)
{                                  
    WAVState *wav = opaque;        
    char *path = wav->path;        
                                   
    monitor_printf (cur_mon, "Capturing audio(%d,%d,%d) to %s: %d bytes\n",
                    wav->freq, wav->bits, wav->nchannels,       
                    path ? path : "<not available>", wav->bytes);
}