KVM: remove the unused parameter of gfn_to_pfn_memslot

The parameter, 'kvm', is not used in gfn_to_pfn_memslot, we can happily remove
it

Signed-off-by: Xiao Guangrong <xiaoguangrong@linux.vnet.ibm.com>
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>