tests/vm/Makefile.include used to assume that it could run in an unconfigured
source tree, and Cirrus CI relies on that.  It was however broken by commit
f4c66f17 ("tests: use tests/venv to run basevm.py-based scripts", 2022-06-06),
which co-opted the virtual environment being used by avocado tests
to also run the basevm.py tests.

For now, reintroduce the usage of qemu.qmp from the source directory, but
without the sys.path() hacks.  The CI configuration can be changed to
install the package via pip when qemu.qmp is removed from the source tree.

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