Anonymous and file-backed RAM allocation are now almost exactly the same.

Reduce code duplication by moving RAM mmap code out of oslib-posix.c and
exec.c.
Reported-by: NMarc-André Lureau <mlureau@redhat.com>
Signed-off-by: NMichael S. Tsirkin <mst@redhat.com>
Reviewed-by: NPaolo Bonzini <pbonzini@redhat.com>
Acked-by: NPaolo Bonzini <pbonzini@redhat.com>
Tested-by: NThibaut Collet <thibaut.collet@6wind.com>