qemu_domain: move video validation out of qemu_command

qemu_domain: move video validation out of qemu_command

All definition validation that doesn't depend on qemu capabilities
and was allowed previously as valid definition should be placed into
qemuDomainDefValidate.

The check whether video type is supported or not was based on an enum
that translates type into model.  Use switch to ensure that if new
video type is added, it will be properly handled.

Signed-off-by: Pavel Hrdina <phrdina@redhat.com>