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 <39929b5b86d72d51fceac59b52303e3fe8a3083e@suse.com>

crobinso/libvirt,VenkatDatta/libvirt,libvirt/libvirt,eskultety/libvirt,datto/libvirt,eskultety/libvirt,nertpinx/libvirt,jfehlig/libvirt,olafhering/libvirt,andreabolognani/libvirt,fabianfreyer/libvirt,datto/libvirt,eskultety/libvirt,fabianfreyer/libvirt,libvirt/libvirt,andreabolognani/libvirt,jardasgit/libvirt,zippy2/libvirt,andreabolognani/libvirt,fabianfreyer/libvirt,jfehlig/libvirt,datto/libvirt,libvirt/libvirt,jardasgit/libvirt,zippy2/libvirt,olafhering/libvirt,VenkatDatta/libvirt,crobinso/libvirt,jfehlig/libvirt,nertpinx/libvirt,olafhering/libvirt,fabianfreyer/libvirt,taget/libvirt,taget/libvirt,jardasgit/libvirt,nertpinx/libvirt,fabianfreyer/libvirt,olafhering/libvirt,andreabolognani/libvirt,datto/libvirt,jardasgit/libvirt,datto/libvirt,taget/libvirt,libvirt/libvirt,nertpinx/libvirt,taget/libvirt,jfehlig/libvirt,eskultety/libvirt,taget/libvirt,crobinso/libvirt,nertpinx/libvirt,andreabolognani/libvirt,VenkatDatta/libvirt,zippy2/libvirt,crobinso/libvirt,eskultety/libvirt,VenkatDatta/libvirt,VenkatDatta/libvirt,zippy2/libvirt,jardasgit/libvirt