pygrub: Add password support

pygrub: Add password support

It basically checks for the presence of password line in grub.conf
of the guest image and if this line is present, it supports both clear
text and md5 versions of the password. Editing the grub entries and
command-line are disabled when some password is set in domain's
grub.conf file but the password was not entered yet. Also, new option
to press 'p' in interactive pygrub has been added to allow entering
the grub password. It's been tested on x86_64 with PV guests and was
working fine. Also, the countdown has been stopped after key was
pressed, ie. the user is probably editing the boot configuration.

Signed-off-by: Michal Novotny <minovotn@redhat.com>