configure: protect against escaping venv when running Meson

If neither --python nor --meson are specified, Meson's generated
build.ninja will invoke Python script using the interpreter *that Meson
itself is running under*; not the one identified by configure.

This is only an issue if Meson's Python interpreter is not "the first
one in the path", which is the one that is used if --python is not
specified.  A common case where this happen is when the "python3" binary
comes from a virtual environment but Meson is not installed (with pip)
in the virtual environment.  In this case (presumably) whoever set up
the venv wanted to use the venv's Python interpreter to build QEMU,
while Meson might use a different one, for example an enterprise
distro's older runtime.

So, detect whether a virtual environment is setup, and if the virtual
environment does not have Meson, use the meson submodule.  Meson will
then run under the virtual environment's Python interpreter.

Reported-by: John Snow <jsnow@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>