block/vhdx: check for offset overflow to bdrv_truncate()

VHDX uses uint64_t types for most offsets, following the VHDX spec.
However, bdrv_truncate() takes an int64_t value for the truncating
offset.  Check for overflow before calling bdrv_truncate().

While we are here, replace the bit shifting with QEMU_ALIGN_UP as well.

N.B.: For a compliant image this is not an issue, as the maximum VHDX
image size is defined per the spec to be 64TB.

Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Jeff Cody <jcody@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
