This patch allows the use a host_device as the destination for "qemu-img
convert".
I added a ->bdrv_create function host_device. It merely verifies that
the device exists and is large enough.
A check is needed in the qemu-img convert loop to ensure that we write
out all 0 sectors to the host_device. Otherwise they end up with stale
garbage where all zero sectors were expected.
I also made the check against bdrv_is_allocated enabled for everything
_except_ host devices, since there is no point in making the block
backend write a bunch of zeros just so that we can memcmp them
immediately afterwards. Host devices can't benefit from this because
there is no way to differentiate between a sector being unallocated
because it was never written, or because it was written with all zeros
and then made a trip through qemu-img convert.
Finally, there is an unrelated fix for a typo in the error message
printed if the destination device does not support ->bdrv_create.
Signed-off-by: Nolan Leake <nolan <at> sigbus.net>