[GuestMemory] change with_regions_mut() signature
[GuestMemory] change with_regions_mut() signature

Signed-off-by: Serban Iorga <4cd75ae1c2f5ea126376f5daa1ce9f5f3016cb24@amazon.com>

firecracker-microvm/firecracker,firecracker-microvm/firecracker,firecracker-microvm/firecracker,firecracker-microvm/firecracker,firecracker-microvm/firecracker