arm/arm64: Fail if RAM size is too large for 32-bit guests

For 64-bit guests, kvmtool exists with an error in kvm__get_vm_type() if
the memory size is larger than what KVM supports. For 32-bit guests, the
RAM size is silently rounded down to ARM_LOMAP_MAX_MEMORY in
kvm__arch_init().

Be consistent and exit with an error when the user has configured the
wrong RAM size for 32-bit guests.

Signed-off-by: Alexandru Elisei <alexandru.elisei@arm.com>