qemu: Don't use NULL path from qemuDomainGetHostdevPath

qemu: Don't use NULL path from qemuDomainGetHostdevPath

Commit v5.10.0-290-g3a4787a301 refactored qemuDomainGetHostdevPath to
return a single path rather than an array of paths. When the function is
called on a missing device, it will now return NULL in @path rather than
a NULL array with zero items and the callers need to be adapted
properly.

Signed-off-by: Jiri Denemark <jdenemar@redhat.com>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>