We are gradually moving away from sector-based interfaces, towards
byte-based.  Make the change for the last few sector-based callbacks
in the vxhs driver.

Note that the driver was already using byte-based calls for
performing actual I/O, so this just gets rid of a round trip
of scaling; however, as I don't know if VxHS is tolerant of
non-sector AIO operations, I went with the conservative approach
of adding .bdrv_refresh_limits to override the block layer
defaults back to the pre-patch value of 512.

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