block/vpc.c: Handle write failures in get_image_offset()

Coverity (CID 1355236) points out that get_image_offset() doesn't check that
it actually succeeded in writing the updated block bitmap to the file.
Check the error return from bdrv_pwrite_sync() and propagate an error
response back up to the function which calls get_image_offset() for
a write so that it can return the error to its caller.

get_sector_offset() is only used for reads, but we move it to the
same API for consistency.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>