KVM: x86: work on all available address spaces

This patch has no semantic change, but it prepares for the introduction
of a second address space for system management mode.

A new function x86_set_memory_region (and the "slots_lock taken"
counterpart __x86_set_memory_region) is introduced in order to
operate on all address spaces when adding or deleting private
memory slots.

Reviewed-by: Radim Krčmář <rkrcmar@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>