qemu: Dissolve qemuBuildVhostuserCommandLine in qemuBuildInterfaceCommandLine

qemu: Dissolve qemuBuildVhostuserCommandLine in qemuBuildInterfaceCommandLine

https://bugzilla.redhat.com/show_bug.cgi?id=1524230

The qemuBuildVhostuserCommandLine builds command line for
vhostuser type interfaces. It is duplicating some code of the
function it is called from (qemuBuildInterfaceCommandLine)
because of the way it's called. If we merge it into the caller
not only we save a few lines but we also enable checks that we
would have to duplicate otherwise (e.g. QoS availability).

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Erik Skultety <eskultet@redhat.com>