qemu: Return -EINVAL to keep qemuDomainOpenFile() consistent

qemu: Return -EINVAL to keep qemuDomainOpenFile() consistent

The description of the function says that the return value is a
file descriptor on success and negative errno on failure which is
not true. If the 'if' case with check on security labels fails,
the return value is -1 not -errno. The solution is to return
'-EINVAL' instead.

Signed-off-by: Kristina Hanicova <khanicov@redhat.com>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>