metadata: track title edits across libvirtd restart

metadata: track title edits across libvirtd restart

https://bugzilla.redhat.com/show_bug.cgi?id=1122205

Although the edits were changing in-memory XML, it was not flushed
to disk; so unless some other action changes XML, a libvirtd restart
would lose the changed information.

* src/conf/domain_conf.c (virDomainObjSetMetadata): Add parameter,
to save live status across restarts.
(virDomainSaveXML): Allow for test driver.
* src/conf/domain_conf.h (virDomainObjSetMetadata): Adjust
signature.
* src/bhyve/bhyve_driver.c (bhyveDomainSetMetadata): Adjust caller.
* src/lxc/lxc_driver.c (lxcDomainSetMetadata): Likewise.
* src/qemu/qemu_driver.c (qemuDomainSetMetadata): Likewise.
* src/test/test_driver.c (testDomainSetMetadata): Likewise.

Signed-off-by: Eric Blake <eblake@redhat.com>