libxl: don't attempt to probe a non-existent emulator

libxl: don't attempt to probe a non-existent emulator

When probing the <emulator> with '-help' to determine if
it is the old qemu, errors are reported if the emulator
doesn't exist

libvirt:  error : internal error: Child process
(/usr/lib/xen/bin/qemu-dm -help) unexpected exit status 127:
libvirt:  error : cannot execute binary /usr/lib/xen/bin/qemu-dm:
No such file or directory

Avoid the probe if the specified emulator doesn't exist,
squelching the error. There is no behavior change since
libxlDomainGetEmulatorType() would return
LIBXL_DEVICE_MODEL_VERSION_QEMU_XEN if the probe failed
via virCommandRun().

Signed-off-by: Jim Fehlig <jfehlig@suse.com>