KVM: x86/xen: Remove extra unlock in kvm_xen_hvm_set_attr()

This accidentally ended up locking and then immediately unlocking kvm->lock
at the beginning of the function. Fix it.

Fixes: a76b9641

 ("KVM: x86/xen: add KVM_XEN_HVM_SET_ATTR/KVM_XEN_HVM_GET_ATTR")
Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>
Message-Id: <20210208232326.1830370-2-dwmw2@infradead.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>