Add try/except for os_dep.command failure

The function os_dep.command() should be surrounded by try/except.
Merge pull request #356 from PandaWei/virt-cmd

Add try/except for os_dep.command failure
virsh_iface_bridge: add checkpoints for iface-bridge test

add two checkpoints for the test of iface-bridge command
1) The new bridge's IP should keep the same as original interface' IP.
2) If '--no-stp' option isn't specified, the STP should be enabled.