qvm-shutdown: Allow multiple VMs to be shut down with one invocation
qvm-shutdown: Allow multiple VMs to be shut down with one invocation

Prior to this commit, qvm-shutdown had a limitation where only one
VM name could be passed in via the command line for shutting down.

This commit removes the aforementioned limitation by adapting the
code for multiple command line arguments.

Signed-off-by: M. Vefa Bicakci <009d335bcb8b986ccbf41b9463116d45753e9f7d@runbox.com>

marmarek/qubes-core-admin,woju/qubes-core-admin,woju/qubes-core-admin,QubesOS/qubes-core-admin,woju/qubes-core-admin,QubesOS/qubes-core-admin,woju/qubes-core-admin,marmarek/qubes-core-admin,QubesOS/qubes-core-admin,marmarek/qubes-core-admin