qemu: default video device type selection algoritm moved into its own function

qemu: default video device type selection algoritm moved into its own function

The default video device type selection algorithm we're about to deploy will
increase the amount of code dedicated to the task by amount enough to warrant
factoring the whole thing into its own function so as not to pollute the
caller qemuDomainDeviceVideoDefPostParse().  Do it now so that the actual
algorithm change later on is in a clean commit by itself and easy to review.

Reviewed-by: Cole Robinson <crobinso@redhat.com>
Signed-off-by: Pavel Mores <pmores@redhat.com>