hvm, x86: Add QEMU BIOS menu for choosing boot device.

hvm, x86: Add QEMU BIOS menu for choosing boot device.

The attached patch implements a mini BIOS menu for choosing a
non-default
boot device. When a guest starts it'll display

   'Press F10 to select boot device'

And wait 3 seconds, before continuing with the normal boot device. If
they press the F10 key, a menu is shown allowing a choice between
floppy, harddisk, cdrom and network (PXE).

I can't take credit for this originally - Jeremy Katz wrote it for
KVM, I merely re-diffed the patch to work against Xen's QEMU/BIOS code
tree. It has been tested in Fedora successfully against 3.1.x and 3.2.x

Signed-off-by: Daniel P. Berrange <berrange@redhat.com>
CC: Jeremy Katz <katzj@redhat.com>
If more than one boot device has been selected, the currently selected
number displayed in the menu is wrong.

Signed-off-by: Stefano Stabellini <stefano.stabellini@citrix.com>