virsh: Complete multiple options when any one option requires data

virsh: Complete multiple options when any one option requires data

Before this patch:
    virsh # start --domain dom1 [TAB][TAB] <- offers filename completion
    virsh # start --domain [TAB][TAB] <- offers filename completion

After this patch:
    virsh # start --domain dom1 [TAB][TAB] <- offers command completion
    virsh # start --domain [TAB][TAB] <- calls domain completer if
    defined, otherwise falls back to filename completion

Signed-off-by: Nishith Shah <nishithshah.2211@gmail.com>