xmlrpcTests: Detect Ubuntu initramfs when booting VM

Ubuntu initramfs file name is different from that in Fedora and RHEL.
This patch tries to detect possible initramfs file names.

Change-Id: I3d8754a297acb3dedbee51270b025eb284617506
Signed-off-by: Zhou Zheng Sheng <zhshzhou@linux.vnet.ibm.com>
Reviewed-on: http://gerrit.ovirt.org/17810
Reviewed-by: Martin Polednik <mpoledni@redhat.com>
Reviewed-by: Dan Kenigsberg <danken@redhat.com>