tools: add configure option for disabling pygrub

tools: add configure option for disabling pygrub

Add a "--disable-pygrub" option for being able to disable the build
and installation of pygrub.

There are two main reasons to do so:

- A main reason to use pygrub is to allow a PV guest to choose its
  bitness (32- or 64-bit). Pygrub allows that by looking into the boot
  image and to start the guest in the correct mode depending on the
  kernel selected. With 32-bit PV guests being deprecated and the
  possibility to even build a hypervisor without 32-bit PV support,
  this use case is gone for at least some configurations.

- Pygrub is running in dom0 with root privileges. As it is operating
  on guest controlled data (the boot image) and taking decisions based
  on this data, there is a higher security risk. Not being possible
  to use pygrub is thus a step towards a reduction of attack surface.

Default is still to build and install pygrub.

Signed-off-by: Juergen Gross <jgross@suse.com>
Acked-by: Anthony PERARD <anthony.perard@citrix.com>