qemu_domain: Drop ATTRIBUTE_NONNULL() for non-existent arguments

qemu_domain: Drop ATTRIBUTE_NONNULL() for non-existent arguments

After cleanup done in v8.2.0-rc1~47 the
qemuDomainObjExitMonitor() and after v8.7.0-rc1~176 the
qemuDomainObjEnterMonitor() lost the @driver argument. But
corresponding ATTRIBUTE_NONNULL() annotation was not removed and
both functions are still annotated as ATTRIBUTE_NONNULL(2) even
though they accept just one argument (@obj).

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