run.py: without -e, revert to the default command line.

As issue #128 explains, it is annoying that once the "-e" option was used
in run.py, it overrides the default command line, so that running run.py
again without -e reruns the same modified command line, and not the default
one as usual.

This simple patch changes this behavior: On every run of run.py, the command
line is set - if "-e" is given it is set to the given command line, but if
"-e" is absent, the command line is reset to the default one. This is
basically the behavior that people always wanted from run.py.

This patch, and the behavior it performs, are much simpler than any of
the alternatives proposed in issue #128.

Please don't complain that this can only be used for development, and not
for EC2, for creating images to distribute, and so on. After all, this is a
patch to *run.py*, and it is obviously not relevant to anyone who is running
an OSv image not through run.py. This patch only makes run.py more convenient
to use.

Fixes #128.

Signed-off-by: Nadav Har'El <nyh@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>
run.py: Add -V (--verbose) option

The OSv loader has a "--verbose" option to enable debug() output to be
printed to the console and not just to a memory buffer. However, needing to
pass this option in the "-e" string is annoying, and even worse when you want
to use the default command line and didn't even want an explict "-e" option.

So this patch adds a --verbose to run.py, which can be used whether or not
a "-e" option is given. For example:

    run.py --verbose
or
    run.py --verbose tests/tst-timerfd.so

As a shorthand for --verbose, I used "-V". Unfortunately, lowercase "-v"
was already taken (by the "vhost" option).

Fixes #173.

Signed-off-by: Nadav Har'El <nyh@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>