xics-kvm: Fix reset function

Currently interrupt priorities are set to 0 (highest) at the very
beginning of the guest execution which is not correct and makes the guest
produce random interrupt error messages such as:
"Interrupt 0x1001 (real) is invalid, disabling it".
This also prevents interrupt states from correct migration.

This initializes priority to 0xFF as the emulated XICS does.

Suggested-by: Paul Mackerras <paulus@samba.org>
Signed-off-by: Alexey Kardashevskiy <aik@ozlabs.ru>
Signed-off-by: Andreas Färber <afaerber@suse.de>
