tools/kvm_stat: switch to argparse

optparse is deprecated for a while, hence switching over to argparse
(which also works with python2).
As a consequence, help output has some subtle changes, the most
significant one being that the options are all listed explicitly
instead of a universal '[options]' indicator. Also, some of the error
messages are phrased slightly different.
While at it, squashed a number of minor PEP8 issues.

Signed-off-by: Stefan Raspl <raspl@linux.ibm.com>
Message-Id: <20200306114250.57585-3-raspl@linux.ibm.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>