virtio-gpu: Don't require udmabuf when blob support is enabled

Host blobs don't need udmabuf, it's only needed by guest blobs. The host
blobs are utilized by the Mesa virgl driver when persistent memory mapping
is needed by a GL buffer, otherwise virgl driver doesn't use blobs.
Persistent mapping support bumps GL version from 4.3 to 4.5 in guest.
Relax the udmabuf requirement.

Signed-off-by: Dmitry Osipenko <dmitry.osipenko@collabora.com>