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>