This changes the helpers qemu{Add,Remove}SharedDisk into
qemu{Add,Remove}SharedDevice, as most of the code in the helpers
can be reused for scsi host device.

To track the shared scsi host device, first it finds out the
device path (e.g. /dev/s[dr]*) which is mapped to the sg device,
and use device ID of the found device path (/dev/s[dr]*) as the
hash key. This is because of the device ID is not unique between
between /dev/s[dr]* and /dev/sg*, e.g.

% sg_map
/dev/sg0  /dev/sda
/dev/sg1  /dev/sr0

% ls -l /dev/sda
brw-rw----. 1 root disk 8, 0 May  2 19:26 /dev/sda

%ls -l /dev/sg0
crw-rw----. 1 root disk 21, 0 May  2 19:26 /dev/sg0