net/virtio: support power monitor

net/virtio: support power monitor

According to current semantics of power monitor, this commit adds a
callback function to decide whether aborts the sleep by checking
current value against the expected value and virtio_get_monitor_addr
to provide address to monitor. When no packet come in, the value of
address will not be changed and the running core will sleep. Once
packets arrive, the value of address will be changed and the running
core will wakeup.

Signed-off-by: Miao Li <miao.li@intel.com>
Reviewed-by: Chenbo Xia <chenbo.xia@intel.com>
Acked-by: David Hunt <david.hunt@intel.com>