mkvenv: add --diagnose option to explain "ensure" failures

mkvenv: add --diagnose option to explain "ensure" failures

This is a routine that is designed to print some usable info for human
beings back out to the terminal if/when "mkvenv ensure" fails to locate
or install a package during configure time, such as meson or sphinx.

Since we are requiring that "meson" and "sphinx" are installed to the
same Python environment as QEMU is configured to build with, this can
produce some surprising failures when things are mismatched. This method
is here to try and ease that sting by offering some actionable
diagnosis.

Signed-off-by: John Snow <jsnow@redhat.com>
Message-Id: <20230511035435.734312-8-jsnow@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>