vfio: enable 8-byte reads/writes to vfio

vfio: enable 8-byte reads/writes to vfio

This patch enables 8-byte writes and reads to VFIO. Such implemention
is already done but it's missing the 'case' to handle such accesses in
both vfio_region_write and vfio_region_read and the MemoryRegionOps:
impl.max_access_size and impl.min_access_size.

After this patch, 8-byte writes such as:

qemu_mutex_lock locked mutex 0x10905ad8
vfio_region_write  (0001:03:00.0:region1+0xc0, 0x4140c, 4)
vfio_region_write  (0001:03:00.0:region1+0xc4, 0xa0000, 4)
qemu_mutex_unlock unlocked mutex 0x10905ad8

goes like this:

qemu_mutex_lock locked mutex 0x10905ad8
vfio_region_write  (0001:03:00.0:region1+0xc0, 0xbfd0008, 8)
qemu_mutex_unlock unlocked mutex 0x10905ad8

Signed-off-by: Jose Ricardo Ziviani <joserz@linux.vnet.ibm.com>
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>