common/vm_event: prevent guest locking with large max_vcpus

common/vm_event: prevent guest locking with large max_vcpus

It is currently possible for the guest to lock when subscribing
to synchronous vm_events if max_vcpus is larger than the
number of available ring buffer slots. This patch no longer
blocks already paused VCPUs, fixing the issue for this use
case, and wakes up as many blocked VCPUs as there are slots
available in the ring buffer, eliminating the blockage for
asynchronous events.

Signed-off-by: Razvan Cojocaru <rcojocaru@bitdefender.com>
Acked-by: Tamas K Lengyel <tamas@tklengyel.com>