tests: replace check-venv with vm-venv target

With the qemu.qmp and qemu.machine dependencies now installed by default
at configure time and additional dependencies required by functional
testing installed on demand, we do not need the explicit "check-venv" target.

...However, to facilitate running VM tests without running configure, we
move some of the former logic into tests/vm/Makefile.include to create a
new venv (vm-venv) on-demand when running VM tests from the source tree.

Reviewed-by: Thomas Huth <thuth@redhat.com>
Message-ID: <20260218213416.674483-17-jsnow@redhat.com>
Signed-off-by: John Snow <jsnow@redhat.com>