vfio: Destroy memslot when unmapping the associated VAs

When we want to map a device region into the guest address space, first we
perform an mmap on the device fd. The resulting VMA is a mapping between
host userspace addresses and physical addresses associated with the device.
Next, we create a memslot, which populates the stage 2 table with the
mappings between guest physical addresses and the device physical adresses.

However, when we want to unmap the device from the guest address space, we
only call munmap, which destroys the VMA and the stage 2 mappings, but
doesn't destroy the memslot and kvmtool's internal mem_bank structure
associated with the memslot.

This has been perfectly fine so far, because we only unmap a device region
when we exit kvmtool. This is will change when we add support for
reassignable BARs, and we will have to unmap vfio regions as the guest
kernel writes new addresses in the BARs. This can lead to two possible
problems:

- We refuse to create a valid BAR mapping because of a stale mem_bank
  structure which belonged to a previously unmapped region.

- It is possible that the mmap in vfio_map_region returns the same address
  that was used to create a memslot, but was unmapped by vfio_unmap_region.
  Guest accesses to the device memory will fault because the stage 2
  mappings are missing, and this can lead to performance degradation.

Let's do the right thing and destroy the memslot and the mem_bank struct
associated with it when we unmap a vfio region. Set host_addr to NULL after
the munmap call so we won't try to unmap an address which is currently used
by the process for something else if vfio_unmap_region gets called twice.

Signed-off-by: Alexandru Elisei <alexandru.elisei@arm.com>
Signed-off-by: Andre Przywara <andre.przywara@arm.com>