vhdx: Rework truncation logic

vhdx: Rework truncation logic

The vhdx driver uses truncation for image growth, with a special case
for blocks that already read as zero but which are only being
partially written.  But with a bit of rearranging, it's just as easy
to defer the decision on whether truncation resulted in zeroes to the
actual allocation attempt, reducing the number of places that still
use bdrv_has_zero_init_truncate.

Signed-off-by: Eric Blake <eblake@redhat.com>
Message-Id: <20200428202905.770727-9-eblake@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>