Detect when Debian is used to launch libvirtd

this patch checks if os_VENDOR is ubuntu to launch
libvirt-bin service. Previously, is_ubuntu() was used, but
this function only detects if a deb packaging is used, so
there were no distinction between Ubuntu and Debian.

Change-Id: I222b71962f49896063910ff2a25e4f57be4bf819
Closes-Bug: 1361260
Merge "Detect when Debian is used to launch libvirtd"