backend: Add RebootVmCommand

Added RebootVmCommand and corresponding RebootVmVDSCommand
that utilizes the new boolean parameter of VDSM shutdown verb.

Note: Currently, the VDSM will only attempt to perform reboot using
guest-agent.

Change-Id: I0693943c01e204da877268c02957f3b936cf3fa2
Signed-off-by: Martin Betak <mbetak@redhat.com>