qga: Add guest-fsfreeze-freeze-list command

If an array of mount point paths is specified as 'mountpoints' argument
of guest-fsfreeze-freeze-list, qemu-ga will only freeze the file systems
mounted on specified paths in Linux guests. Otherwise, it works as the
same way as guest-fsfreeze-freeze.
This would be useful when the host wants to create partial disk snapshots.

Signed-off-by: Tomoki Sekiyama <tomoki.sekiyama@hds.com>
Reviewed-by: Eric Blake <eblake@redhat.com>

*updated schema to report 2.2 as initial supported version

Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>