KVM: set_memory_region: Drop user_alloc from prepare/commit_memory_region()

X86 does not use this any more.  The remaining user, s390's !user_alloc
check, can be simply removed since KVM_SET_MEMORY_REGION ioctl is no
longer supported.

Note: fixed powerpc's indentations with spaces to suppress checkpatch
errors.

Signed-off-by: Takuya Yoshikawa <yoshikawa_takuya_b1@lab.ntt.co.jp>
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>