qga: add command guest-get-disks

qga: add command guest-get-disks

Add API and stubs for new guest-get-disks command.

The command guest-get-fsinfo can be used to list information about disks
and partitions but it is limited only to mounted disks with filesystem.
This new command should allow listing information about disks of the VM
regardles whether they are mounted or not. This can be usefull for
management applications for mapping virtualized devices or pass-through
devices to device names in the guest OS.

Signed-off-by: Tomáš Golembiovský <tgolembi@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Signed-off-by: Michael Roth <michael.roth@amd.com>