qemu: tighten semantics of 'size' when resizing block devices

qemu: tighten semantics of 'size' when resizing block devices

When VIR_DOMAIN_BLOCK_RESIZE_CAPACITY is set, the 'size' parameter
is currently ignored. Since applications must none the less pass a
value for this parameter, it is preferrable to declare some explicit
semantics for it.

This declare that the parameter must be 0, or the exact size of the
underlying block device. The latter gives the management application
the ability to sanity check that the block device size matches what
they think it should be.

Reviewed-by: Peter Krempa <pkrempa@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>