xen/arm: vsysreg: Add wrapper to handle sysreg access trapped by HCR_EL2.TVM

xen/arm: vsysreg: Add wrapper to handle sysreg access trapped by HCR_EL2.TVM

A follow-up patch will require to emulate some accesses to system
registers trapped by HCR_EL2.TVM. When set, all NS EL1 writes to the
virtual memory control registers will be trapped to the hypervisor.

This patch adds the infrastructure to passthrough the access to the host
registers.

Note that HCR_EL2.TVM will be set in a follow-up patch dynamically.

Signed-off-by: Julien Grall <julien.grall@arm.com>
Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>