KVM: sefltests: Add explicit synchronization to move mem region test

Use sem_post() and sem_timedwait() to synchronize test stages between
the vCPU thread and the main thread instead of using usleep() to wait
for the vCPU thread and hoping for the best.

Opportunistically refactor the code to make it suck less in general,
and to prepare for adding more testcases.

Suggested-by: Peter Xu <peterx@redhat.com>
Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
Message-Id: <20200410231707.7128-6-sean.j.christopherson@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>