tests: use tests/venv to run basevm.py-based scripts

tests: use tests/venv to run basevm.py-based scripts

This patch co-opts the virtual environment being used by avocado tests
to also run the basevm.py tests. This is being done in preparation for
for the qemu.qmp package being removed from qemu.git.

As part of the change, remove any sys.path() hacks and treat "qemu" as a
normal third-party import.

Signed-off-by: John Snow <jsnow@redhat.com>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Message-Id: <20220526000921.1581503-8-jsnow@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>