launch: direct: Supply -M option when testing qemu for devices.

qemu-system-arm 2.0 as currently available through Debian/unstable
doesn't want to tell us anything about available devices unless we
specify a machine type.
launch: direct: Turn gdb into a backend setting.

You can now use LIBGUESTFS_BACKEND_SETTINGS=gdb in order to enable gdb
debugging of the appliance.  This only works with the direct backend.
builder: Add RHEL 7 signature to EXTRA_DIST.

This fixes commit fea553778a1f38b8c0dc3bf76a67725f0cee9094.
Version 1.27.8.