KVM: arm64: Return error from kvm_arch_init_vm() on allocation failure

If we fail to allocate the 'supported_cpus' cpumask in kvm_arch_init_vm()
then be sure to return -ENOMEM instead of success (0) on the failure
path.

Reviewed-by: Alexandru Elisei <alexandru.elisei@arm.com>
Signed-off-by: Will Deacon <will@kernel.org>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20220609121223.2551-2-will@kernel.org