mkvenv: pass first missing package to diagnose()

mkvenv: pass first missing package to diagnose()

If sphinx is present but the theme is not, mkvenv will print an
inaccurate diagnostic:

ERROR: Could not find a version that satisfies the requirement sphinx-rtd-theme>=0.5.0 (from versions: none)
ERROR: No matching distribution found for sphinx-rtd-theme>=0.5.0

'sphinx>=1.6.0' not found:
 • Python package 'sphinx' version '5.3.0' was found, but isn't suitable.
 • mkvenv was configured to operate offline and did not check PyPI.

Instead, ignore the packages that were found to be present, and report
an error based on the first absent package.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>