Adds 'quiesced' status into qemuDomainObjPrivate that tracks whether
FSFreeze is requested in the domain.

It modifies error code from qemuDomainSnapshotFSFreeze and
qemuDomainSnapshotFSThaw, so that a caller can know whether the command is
actually sent to the guest agent. If the error is caused before sending a
freeze command, a counterpart thaw command shouldn't be sent either, not to
confuse fsfreeze status tracking.
Signed-off-by: NTomoki Sekiyama <tomoki.sekiyama@hds.com>
Signed-off-by: NEric Blake <eblake@redhat.com>