KVM: arm64: Factor out KVM_GET_REG_LIST core register enumeration

Currently, enumeration of the core register IDs for
KVM_GET_REG_LIST is open-coded in kvm_arm_copy_reg_indices().

This will become cumbersome as the enumeration logic becomes more
complex.  In preparation for future patches, this patch factors the
code out into a separate function copy_core_reg_indices(),
consistently with the way other classes of registers are handled.

No functional change.

Signed-off-by: Dave Martin <Dave.Martin@arm.com>