KVM: arm64: Fix WFxT handling of nested virt

KVM: arm64: Fix WFxT handling of nested virt

The spec for WFxT indicates that the parameter to the WFxT instruction
is relative to the reading of CNTVCT_EL0. This means that the implementation
needs to take the execution context into account, as CNTVOFF_EL2
does not always affect readings of CNTVCT_EL0 (such as when HCR_EL2.E2H
is 1 and that we're in host context).

This also rids us of the last instance of KVM_REG_ARM_TIMER_CNT
outside of the userspace interaction code.

Signed-off-by: Marc Zyngier <maz@kernel.org>