arm64/vmm: save and restore HCRX_EL2 register

With the addition of the Extended Hypervisor Configuration Register
(HCRX_EL2), this change ensures that it is both set to 0 before entering
a vm, and that it is properly saved/restored.

Reviewed by: andrew
Sponsored by: Arm Ltd
Differential Revision: https://reviews.freebsd.org/D48584
Signed-off-by: Harry Moulton <harry.moulton@arm.com>