package/qemu: build for the target (i386 or x86_64 only)

Untested for other architectures, so only enabled for known-working ones.

We explicitly disable all features that depends on other packages,
for now. Such features will be added one by one in the next patches.
Features that do not rely on external packages are explicitly enabled
to catch the dependencies on toolchain features early.

[Peter: propagate dependencies from libglib2]
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>