migration: Fix global state with Xen.

When doing migration via the QMP command xen_save_devices_state, the
current runstate is not store into the global state section. Also the
current runstate is not the one we want on the receiver side.

During migration, the Xen toolstack paused QEMU before save the devices
state. Also, the toolstack expect QEMU to autostart when the migration is
finished.
So this patch store "running" as it's current runstate.

Signed-off-by: Anthony PERARD <anthony.perard@citrix.com>
Signed-off-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>