qemu_monitor_json: add support to search QOM device path by device alias

qemu_monitor_json: add support to search QOM device path by device alias

Commit ce745914 introduced detection of actual video ram sizes to fix migration
if QEMU decide to modify the values provided by libvirt.  This works perfectly
for domains with number of video devices up to two.

If there are more than two video devices in the guest all the secondary devices
in the XML will have the same memory values.  This is because our current code
search for QOM device path only by the device type name and all the secondary
video devices has the same name "qxl".

This patch introduces a new search function that will try to search a QOM device
path using also device's alias if the alias is available. After that it will
fallback to the old recursive code if the alias search found no results.

Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1358728

Signed-off-by: Pavel Hrdina <phrdina@redhat.com>