util/mmap-alloc: Factor out reserving of a memory region to mmap_reserve()

util/mmap-alloc: Factor out reserving of a memory region to mmap_reserve()

We want to reserve a memory region without actually populating memory.
Let's factor that out.

Reviewed-by: Igor Kotrasinski <i.kotrasinsk@partner.samsung.com>
Acked-by: Murilo Opsfelder Araujo <muriloo@linux.ibm.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Peter Xu <peterx@redhat.com>
Acked-by: Eduardo Habkost <ehabkost@redhat.com> for memory backend and machine core
Signed-off-by: David Hildenbrand <david@redhat.com>
Message-Id: <20210510114328.21835-3-david@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>