vdpa: mask _F_CTRL_GUEST_OFFLOADS for vhost vdpa devices

vdpa: mask _F_CTRL_GUEST_OFFLOADS for vhost vdpa devices

QEMU does not emulate it so it must be disabled as long as the backend
does not support it.

Signed-off-by: Eugenio Pérez <eperezma@redhat.com>
Message-Id: <20230602173328.1917385-1-eperezma@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Acked-by: Jason Wang <jasowang@redhat.com>
Tested-by: Lei Yang <leiyang@redhat.com>
(cherry picked from commit 51e84244a7799172f4239482199e9b4bdcd23172)
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>