configure: Disable guest_agent for mingw32

configure: Disable guest_agent for mingw32

guest_agent is not supported for mingw32, so the default value
should be 'no', not 'yes'.

This removes the dependencies to glib-2.0 and python which
makes native and cross builds for w32 much easier (no need
to get and install these extra packages).

It also avoids the problems caused by different bitfield alignment
which is required by glib-2.0.

It is still possible to set guest_agent=yes via configure option.

Signed-off-by: Stefan Weil <weil@mail.berlios.de>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>