KVM: direct mmio pfn check

Userspace may specify memory slots that are backed by mmio pages rather than
normal RAM.  In some cases it is not enough to identify these mmio pages
by pfn_valid().  This patch adds checking the PageReserved as well.

Signed-off-by: Ben-Ami Yassour <benami@il.ibm.com>
Signed-off-by: Muli Ben-Yehuda <muli@il.ibm.com>
Signed-off-by: Avi Kivity <avi@qumranet.com>