Emulation of the block limits VPD page called back into scsi-disk.c,
which however expected the request to be for a SCSIDiskState and
accessed a scsi-generic device outside the bounds of its struct
(namely to retrieve s->max_unmap_size and s->max_io_size).

To avoid this, move the emulation code to a separate function that
takes a new SCSIBlockLimits struct and marshals it into the VPD
response format.
Reported-by: NMax Reitz <mreitz@redhat.com>
Reviewed-by: NMax Reitz <mreitz@redhat.com>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>