By default QEMU enables a lot of features if it can probe and find the
support libraries. It also enables a bunch of features by default.
This patch adds the ability to build --without-default-features which
can be paired with a --without-default-devices for a barely functional
build.

The main use case for this is testing our build assumptions and for
minimising the amount of stuff you build if you just want to test a
particular feature on your relatively slow emulated test system. On
it's own I go from:

  $ ls -lh qemu-system-aarch64
  -rwxr-xr-x 1 alex alex 120M Dec 10 12:45 qemu-system-aarch64*
  $ ldd qemu-system-aarch64 | wc -l
  170

to:

  $ ls -lh qemu-aarch64
  -rwxr-xr-x 1 alex alex 43M Dec 10 12:41 qemu-aarch64*
  $ ldd qemu-system-aarch64 | wc -l
  57

which is still able to run my default Debian ARM64 machine with a lot
less fat involved.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Acked-by: Thomas Huth <thuth@redhat.com>
Acked-by: Paolo Bonzini <pbonzini@redhat.com>
Message-Id: <20201210190417.31673-5-alex.bennee@linaro.org>