qemu: Define libnfs PACKAGECONFIG

The upstream qemu recipe uses host's pkg-config files as a solution to
detecting host's SDL. This has a side effect of using other host
libraries that are later queried by the configure script. This can get
into a situation when the host provides libnfs (for example) and because
later this dependency is not in place anymore, qemu will fail at
runtime.

This change adds a PACKAGECONFIG definition for libnfs that is disabled
by default, in turn disabling the pkgconfig autodetection in configure.

Signed-off-by: Andrei Gherzan <andrei.gherzan@huawei.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>