virt-libvirt: Simplify libvirt.create() serial

Partial fix for issue 427

Signed-off-by: Satheesh Rajendran <sathnaga@linux.vnet.ibm.com>
Signed-off-by: Chris Evich <cevich@redhat.com>
virt: Move get_serial_*() methods to qemu_vm

This interface is not generally applicable as libvirt
already has a related mechanism.  Moved it to
qemu_vm module and verified it is not called from
any other non-qemu tests.

Signed-off-by: Chris Evich <cevich@redhat.com>
virt-libvirt: Remove unused serial interface

For libvirt testing, we have access to serial consoles
built in to the API and virsh command.  Especially
during unattended_install, it is simply not necessary
to have both a pty type and a file-output type. The
unattended_install simply calls vm.serial_console.get_output()
in all cases, so having the extra log file was wasted.

Signed-off-by: Chris Evich <cevich@redhat.com>