KVM: selftests: Consolidate KVM_{G,S}ET_ONE_REG helpers

Rework vcpu_{g,s}et_reg() to provide the APIs that tests actually want to
use, and drop the three "one-off" implementations that cropped up due to
the poor API.

Ignore the handful of direct KVM_{G,S}ET_ONE_REG calls that don't fit the
APIs for one reason or another.

No functional change intended.

Signed-off-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>