selftests: kvm: move vm_xsave_req_perm call to amx_test

There is no need for tests other than amx_test to enable dynamic xsave
states.  Remove the call to vm_xsave_req_perm from generic code,
and move it inside the test.  While at it, allow customizing the bit
that is requested, so that future tests can use it differently.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>