vm: replace try/finally with 'with'

The creation of VM is throttled using a bounded semaphore.
This patch
* replaces the try/finally construct with 'with' to make
  code (a tiny bit) nicer ad smaller
* drops the log lines around the lock, because
  - we are now guaranteed the code is correct and always
    release the lock
  - if we care about performances and timings (e.g.time spent
    in locked region) we need more precise instrumentation
    anyway.

Change-Id: I1bf75d64ab4e838957cbd2b0890b8cbc6f9fc9b0
Signed-off-by: Francesco Romani <fromani@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/34529
Reviewed-by: Dan Kenigsberg <danken@redhat.com>