bifrost: Do not run virsh commands if it's not installed

bifrost: Do not run virsh commands if it's not installed

Check if 'virsh' is available before trying to remove the VMs:

Fixes the following warning

./scripts/destroy-env.sh: line 36: virsh: command not found
./scripts/destroy-env.sh: line 37: virsh: command not found
./scripts/destroy-env.sh: line 36: virsh: command not found
./scripts/destroy-env.sh: line 37: virsh: command not found
./scripts/destroy-env.sh: line 36: virsh: command not found
./scripts/destroy-env.sh: line 37: virsh: command not found

Change-Id: I9c82c8b8b40462584c6324871284ba12d660ae9d
Signed-off-by: Markos Chandras <mchandras@suse.de>