XML <source bridge='VMnet0'/> update in <interface type='bridge'/>

XML <source bridge='VMnet0'/> update in <interface type='bridge'/>

Previously, we accepted empty bridge name, because some old versions of
VMWare Workstation did not put it into the config. But this doesn't make
much sense - to have an interface type bridge with no name. We
circumvented this problem by generating an empty name but that is
equally wrong.

Therefore, fill in missing bridge names (according to the documentation
[1] the default bridge name is VMnet0) and error out if bridge name is
missing.

This partially reverts f246cdb5aca13ac9409b2ad43087e3078615ffcb

1: https://docs.vmware.com/en/VMware-Workstation-Player-for-Linux/16.0/com.vmware.player.linux.using.doc/GUID-BAFA66C3-81F0-4FCA-84C4-D9F7D258A60A.html

Signed-off-by: Kristina Hanicova <khanicov@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>
Signed-off-by: Ján Tomko <jtomko@redhat.com>