qemuDomainDefPostParse: Fetch qemuCaps from domain object

qemuDomainDefPostParse: Fetch qemuCaps from domain object

We can't rely on def->emulator path. It may be provided by user
as we give them opportunity to provide their own XML for
migration. Therefore the path may point to just whatever binary
(or even to a non-existent file). Moreover, this path is meant
for destination, but the capabilities lookup is done on source.
What we can do is to assume same capabilities for post parse
callbacks as the running domain has. They will be used just to
add some default models/controllers/devices/... anyway.

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