bhyve: support parsing fbuf PCI device

bhyve: support parsing fbuf PCI device

Add a new helper function, bhyveParsePCIFbuf, to parse the bhyve-argv
parameters for a frame-buffer device to <graphics/> and <video/>
definitions.

For now, only the listen address, port, and vga mode are detected.
Unsupported parameters are silently skipped.

This involves upgrading the private API to expose the
virDomainGraphicsDefNew helper function, which is used by
bhyveParsePCIFbuf.

Signed-off-by: Fabian Freyer <fabian.freyer@physik.tu-berlin.de>
Signed-off-by: Roman Bogorodskiy <bogorodskiy@gmail.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>