KVM: selftests: Convert debug_regs away from VCPU_ID

Convert debug_regs to use vm_create_with_one_vcpu() and pass around a
'struct kvm_vcpu' object instead of using a global VCPU_ID.

Opportunstically drop the CLEAR_DEBUG/APPLY_DEBUG macros as they only
obfuscate the code, e.g. operating on local variables not "passed" to the
macro is all kinds of confusing.

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