libxl: set XEN_QEMU_CONSOLE_LIMIT for QEMU

libxl: set XEN_QEMU_CONSOLE_LIMIT for QEMU

XSA-180 provides a patch to QEMU to bodge QEMU logging issue. We
explicitly set the limit in libxl for 4.7.

Introduce a function for setting the environment variable and call it in
the right places.

Signed-off-by: Wei Liu <wei.liu2@citrix.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>
Release-acked-by: Wei Liu <wei.liu2@citrix.com>
(cherry picked from commit b0d409d9c4944ed29d29457fea4ad6b41d698eca)

(cherry picked from commit fe82a96a657f36b08ade60ec4f3a53e67a4ee314)
Conflicts:
	tools/libxl/libxl_dm.c

This version of libxl does not pass a dm_envs to the
*build_device_model_args* functions.  Instead, call
libxl__set_qemu_env_for_xsa_180 in libxl__spawn_local_dm.

The other call ultimate site of *build_device_model_args* (ie of
libxl__build_device_model_args) is in libxl__spawn_stub_dm, where we
don't need to set the env var.

Signed-off-by: Ian Jackson <ian.jackson@eu.citrix.com>