qga: Add 'guest-get-host-name' command

Retrieving the guest host name is a very useful feature for virtual management
systems. This information can help to have more user friendly VM access
details, instead of an IP there would be the host name. Also the host name
reported can be used to have automated checks for valid SSL certificates.

virsh # qemu-agent-command F25 '{ "execute": "guest-get-host-name" }'
{"return":{"host-name":"F25.lab.evilissimo.net"}}

Signed-off-by: Vinzenz Feenstra <vfeenstr@redhat.com>
* minor whitespace fix-ups
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>
