qemu: Relax -no-shutdown check to [0.14.0, 0.15.50)

qemu: Relax -no-shutdown check to [0.14.0, 0.15.50)

SIGTERM handling for -no-shutdown is already fixed in qemu git and
libvirt can safely use it. The downside is that 0.15.50 version of qemu
can be any qemu compiled from git, even that without the fix for
SIGTERM. However, I think this patch is worth it since excluding 0.15.50
from the check makes testing current qemu with libvirt much easier and
someone running qemu from git should be able to rebuild fixed qemu from
git if they hit the problem with a hang on shutdown.