domain_conf: Fix str2enum translation of video driver name

In bc1e924cf0d we've introduced video driver name and whilst
doing so we've utilized VIR_ENUM_IMPL() macro. Then, in domain
XML parsing code the generated
virDomainVideoBackendTypeFromString() is called and its return
value is assigned directly to an unsigned int variable which is
wrong. Also, the video driver enum has 'default' value which is
not formatted into domain XML but is accepted during parsing.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>