qemu_alias: Be more tolerant if alias don't follow our format
    
    When assigning alias to a device we usually iterate over other
    devices of its kind trying to find next index. We do this by
    stripping down the prefix and then parsing number at the end,
    Usually, if the prefix doesn't match the one we are expecting, we
    just continue with next iteration. Except for couple of
    functions: qemuGetNextChrDevIndex(),
    qemuAssignDeviceRedirdevAlias() and qemuAssignDeviceShmemAlias().
    
    Signed-off-by: Michal Privoznik <mprivozn@redhat.com>