virsh: Require explicit --domain for domxml-to-native

The domxml-to-native virsh command accepts either --xml or --domain
option followed by a file or domain name respectively. The --domain
option is documented as required, which means an argument with no option
is treated as --xml. Commit v4.3.0-127-gd86531daf2 broke this by making
--domain optional and thus an argument with no option was treated as
--domain.

https://bugzilla.redhat.com/show_bug.cgi?id=1633077

Signed-off-by: Jiri Denemark <jdenemar@redhat.com>