u_boot_pylib: Use correct coverage tool within venv

When running within a Python venv we must use the 'coverage' tool (which
is within the venv) so that the venv packages are used in preference to
system packages. Otherwise the coverage tests run in a different
environment from the normal tests and may fail due to missing packages.

Handle this by detecting the venv and changing the tool name.

Signed-off-by: Simon Glass <sjg@chromium.org>