KVM: arm64: Support runtime sysreg filtering for KVM_GET_REG_LIST

KVM_GET_REG_LIST should only enumerate registers that are actually
accessible, so it is necessary to filter out any register that is
not exposed to the guest.  For features that are configured at
runtime, this will require a dynamic check.

For example, ZCR_EL1 and ID_AA64ZFR0_EL1 would need to be hidden
if SVE is not enabled for the guest.

Special-casing walk_one_sys_reg() for specific registers will make
the code unnecessarily messy, so this patch adds a new sysreg
method check_present() that, if defined, indicates whether the
sysreg should be enumerated.  If the guest runtime configuration
may require a particular system register to be hidden,
check_present should point to a function that returns true or false
to enable or disable enumeration of that register respectively.

Currently check_present() is not used for any other purpose, but it
may be a useful foundation for abstracting other parts of the code
to handle conditionally-present sysregs, if required.

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