libxl: Support user-specified <emulator>
    
    With the introduction of the libxlDomainGetEmulatorType function,
    it is trivial to support a user-specfied <emulator> in the libxl
    driver.  This patch is based loosely on David Scott's old patch
    to do the same
    
    https://www.redhat.com/archives/libvir-list/2013-April/msg02119.html
    Signed-off-by: Jim Fehlig <jfehlig@suse.com>