Change all virsh commands that invoke virDomain{Attach,Detach}Device()
to use virDomain{Attach,Detach}DeviceFlags() instead.

Add a "--persistent" flag to these virsh commands, allowing user to
specify that the domain persisted config be modified as well.

V2: Only invoke virDomain{Attach,Detach}DeviceFlags() if
"--persistent" flag is specified.  Otherwise invoke
virDomain{Attach,Detach}Device() to retain current behavior.