Use the CPU feature framework to determine whether the WFxT instructions are available on all CPUs, and set enable_wfxt accordingly. When the timer is used for DELAY(), it'll check whether it can use wfet().

Move configuration of hcrx_el2 to avoid trashing cptr_el2

Addressed review comments

Moved HCRX register definition to hypervisor.h, moved initial
configuration of hcrx_el2 to after .Ldone_vhe

In D48584#1108156, @andrew wrote:

Does the value of the register matter when no guest is running? Does the hardware change the register when a guest is running? If not we can restrict when we save & restore it, e.g. if the host doesn't care what value it has, and it's never changed by hardware then we only need to restore it for guests and can drop the vmm_hyp_reg_store change.

Cast to uint64_t not needed, removed.