ARM: KVM: Fix length of mmio access

Instead of hardcoding the maximum MMIO access to be 4 bytes,
compare it to sizeof(unsigned long), which will do the
right thing on both 32 and 64bit systems.

Same thing for sign extention.

Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>
Signed-off-by: Christoffer Dall <cdall@cs.columbia.edu>