kvmvapic: Prevent reading beyond the end of guest RAM

rom_state_paddr is guest provided (caller address of outw(VAPIC_PORT) +
writen 16-bit value) and can be influenced to point beyond the end of
the host memory backing the guest's RAM. Make sure we do not use this
pointer to actually read beyond the limits.

Reading arbitrary guest bytes is harmless, the guest kernel has to
manage access to this I/O port anyway.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
Acked-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Gleb Natapov <gleb@redhat.com>
