Support output type format "qcow"

[abbbi]

During backup, another qemu-nbd process could be mapped to a target qcow image (just like it is happening during restore). The backup data could be directly sent to this process and the resulting backup image would be a standard qcow image, instead of the proprietary "sparsestream" image. This would ease things up during restore.

[abbbi]

POC:

1. start nbd backup service on socket:

./virtnbdbackup -l full -d vm1 -s -o /tmp/bla

2. create qcow with same size:

qemu-img create -f qcow2 test.qcow2 50M

3. copy data via nbdcopy to qcow file:

nbdcopy -p nbd+unix:///sda?socket=/var/tmp/virtnbdbackup.16561 -- [ qemu-nbd -f qcow2 test.qcow2 ]