memory: Rename mem_begin/mem_commit/mem_add helpers

This renames some helpers to reflect better what they do.

This should cause no behavioural change.

Signed-off-by: Alexey Kardashevskiy <aik@ozlabs.ru>
Message-Id: <20170921085110.25598-9-aik@ozlabs.ru>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>