FROMLIST: virt: gunyah: Implement guestmemfd

Memory provided to Gunyah virtual machines are provided by a
Gunyah guestmemfd. Because memory provided to virtual machines may be
unmapped at stage-2 from the host (i.e. in the hypervisor's page tables
for the host), special care needs to be taken to ensure that the kernel
doesn't have a page mapped when it is lent to the guest. Without this
tracking, a kernel panic could be induced  by userspace tricking the
kernel into accessing guest-private memory.

Introduce the basic guestmemfd ops and ioctl. Userspace should be able
to access the memory unless it is provided to the guest virtual machine:
this is necessary to allow userspace to preload binaries such as the
kernel Image prior to running the VM. Subsequent commits will wire up
providing the memory to the guest.

Bug: 338347082
Link: https://lore.kernel.org/all/20240222-gunyah-v17-23-1e9da6763d38@quicinc.com/


Change-Id: Ifa869957b3b0c1e83ac94c44f9cf5acfee8aa4bc
Signed-off-by: Elliot Berman <quic_eberman@quicinc.com>