qga: Add guest-get-fsinfo command

Add command to get mounted filesystems information in the guest.
The returned value contains a list of mountpoint paths and
corresponding disks info such as disk bus type, drive address,
and the disk controllers' PCI addresses, so that management layer
such as libvirt can resolve the disk backends.

For example, when `lsblk' result is:

    NAME           MAJ:MIN RM  SIZE RO TYPE MOUNTPOINT
    sdb              8:16   0    1G  0 disk
    `-sdb1           8:17   0 1024M  0 part
      `-vg0-lv0    253:1    0  1.4G  0 lvm  /mnt/test
    sdc              8:32   0    1G  0 disk
    `-sdc1           8:33   0  512M  0 part
      `-vg0-lv0    253:1    0  1.4G  0 lvm  /mnt/test
    vda            252:0    0   25G  0 disk
    `-vda1         252:1    0   25G  0 part /

where sdb is a SCSI disk with PCI controller 0000:00:0a.0 and ID=1,
      sdc is an IDE disk with PCI controller 0000:00:01.1, and
      vda is a virtio-blk disk with PCI device 0000:00:06.0,

guest-get-fsinfo command will return the following result:

    {"return":
     [{"name":"dm-1",
       "mountpoint":"/mnt/test",
       "disk":[
        {"bus-type":"scsi","bus":0,"unit":1,"target":0,
         "pci-controller":{"bus":0,"slot":10,"domain":0,"function":0}},
        {"bus-type":"ide","bus":0,"unit":0,"target":0,
         "pci-controller":{"bus":0,"slot":1,"domain":0,"function":1}}],
       "type":"xfs"},
      {"name":"vda1", "mountpoint":"/",
       "disk":[
        {"bus-type":"virtio","bus":0,"unit":0,"target":0,
         "pci-controller":{"bus":0,"slot":6,"domain":0,"function":0}}],
       "type":"ext4"}]}

In Linux guest, the disk information is resolved from sysfs. So far,
it only supports virtio-blk, virtio-scsi, IDE, SATA, SCSI disks on x86
hosts, and "disk" parameter may be empty for unsupported disk types.

Signed-off-by: Tomoki Sekiyama <tomoki.sekiyama@hds.com>

*updated schema to report 2.2 as initial supported version

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