vdpa: add get_backend_features vdpa operation

mainline inclusion
from mainline-v6.6-rc1
commit b63e5c70
category: feature
bugzilla: https://gitee.com/openeuler/kernel/issues/I86ITO
Reference: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=b63e5c70c39349ea5b9e7dbb604551902fc753fc



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This operation allow vdpa parent to expose its own backend feature bits.

Next patches introduce a feature not compatible with all parent drivers:
the ability to enable vq after driver_ok.  Each parent must declare if
it allows it or not.

Signed-off-by: Eugenio Pérez <eperezma@redhat.com>
Acked-by: Shannon Nelson <shannon.nelson@amd.com>
Message-Id: <20230609092127.170673-4-eperezma@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: jiangdongxu <jiangdongxu1@huawei.com>