KVM: arm64: Convert vcpu sysregs_loaded_on_cpu to a state flag

The aptly named boolean 'sysregs_loaded_on_cpu' tracks whether
some of the vcpu system registers are resident on the physical
CPU when running in VHE mode.

This is obviously a flag in hidding, so let's convert it to
a state flag, since this is solely a host concern (the hypervisor
itself always knows which state we're in).

Reviewed-by: Fuad Tabba <tabba@google.com>
Reviewed-by: Reiji Watanabe <reijiw@google.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>