vdsm: Replaced all too generic except: handlers in vm.py

Change-Id: I00f1c6650b357cebfcaa375a2babb8c12637fd5a
Signed-off-by: Vinzenz Feenstra <vfeenstr@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/17740
Reviewed-by: Dan Kenigsberg <danken@redhat.com>
Reviewed-by: Francesco Romani <fromani@redhat.com>
Tested-by: Francesco Romani <fromani@redhat.com>