virt-admin: Add warning when connection to default daemon fails

virt-admin: Add warning when connection to default daemon fails

The admin connection defaults to the system-wide 'libvirtd' daemon to
manage (libvirtd:///system). As we've now switched to modular daemons
this will not work for most users out of the box:

 $ virt-admin version
 error: Failed to connect to the admin server
 error: no valid connection
 error: Failed to connect socket to '/run/user/1000/libvirt/libvirt-admin-sock': No such file or directory

As we don't want to assume which daemon the user wants to manage in the
modular topology there's no reasonable default to pick.

Give a hint to the users to use the '-c' if the connection to the
default URI fails:

 $ virt-admin version
 NOTE: Connecting to default daemon. Specify daemon using '-c' (e.g. virtqemud:///system)
 error: Failed to connect to the admin server
 error: no valid connection
 error: Failed to connect socket to '/run/user/1000/libvirt/libvirt-admin-sock': No such file or directory

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Jonathon Jongsma <jjongsma@redhat.com>