qemuDomainDefPostParse: Fail if unable to fill machine type

qemuDomainDefPostParse: Fail if unable to fill machine type

Previously, we used virCapabilitiesDomainDataLookup() to fill
machine type in post parse callback if none was provided in the
domain XML. If machine type couldn't be filled in an error was
reported. After 4a4132b4625 we've changed it to
virQEMUCapsGetPreferredMachine() which returns NULL, but we no
longer report an error and proceed with the post parse callbacks
processing. This may lead to a crash because the code later on
assumes def->os.machine is not NULL.

Fixes: 4a4132b4625
Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Pavel Mores <pmores@redhat.com>