kvm tools: Add a brlock

kvm tools: Add a brlock

brlock is a lock which is very cheap for reads, but very expensive
for writes.
This lock will be used when updates are very rare and reads are
common.
This lock is currently implemented by stopping the guest while
performing the updates. We assume that the only threads which
read from the locked data are VCPU threads, and the only writer
isn't a VCPU thread.

Signed-off-by: Sasha Levin <levinsasha928@gmail.com>
Signed-off-by: Pekka Enberg <penberg@kernel.org>