configure: fix detection of gdbus-codegen

configure: fix detection of gdbus-codegen

"pkg-config --variable=gdbus_codegen gio-2.0" returns "gdbus-codegen",
and it does not pass test -x (which does not walk the path).

Meson 0.58.0 notices that something is iffy, as the dbus_vmstate1
assignment in tests/qtest/meson.build uses an empty string as the
command, and fails very eloquently:

../tests/qtest/meson.build:92:2: ERROR: No program name specified.

Use the "has" function instead of test -x, and fix the generation
of config-host.mak since meson.build expects that GDBUS_CODEGEN
is absent, rather than empty, if the tool is unavailable.

Reported-by: Sebastian Mitterle <smitterl@redhat.com>
Fixes: #178
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>