arm/arm64: Consolidate RAM initialization in kvm__init_ram()

RAM initialization is unnecessarily split between kvm__init_ram() and
kvm__arch_init(). Move all code related to RAM initialization to
kvm__init_ram(), making the code easier to follow and to modify.

One thing to note is that the initialization order is slightly altered:
kvm__arch_enable_mte() and gic__create() are now called before mmap'ing the
guest RAM. That is perfectly fine, as they don't use the host's mapping of
the guest memory.

Signed-off-by: Julien Grall <julien.grall@arm.com>
Signed-off-by: Alexandru Elisei <alexandru.elisei@arm.com>