kvm: Relax aligment check of kvm_set_phys_mem

There is no need to reject an unaligned memory region registration if
the region will be I/O memory and it will not split an existing KVM
slot. This fixes KVM support on PPC.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
