x86/vMSI-x: check whether msixtbl_list in msixtbl_pt_register()

x86/vMSI-x: check whether msixtbl_list in msixtbl_pt_register()

MSI-x tables' initializtion had been deferred in the commit
74c6dc2d0ac4dcab0c6243cdf6ed550c1532b798. If an assigned device does not support
MSI-x, the msixtbl_list won't be initialized. However, the following paths
    XEN_DOMCTL_bind_pt_irq
	pt_irq_create_bind
	    msixtbl_pt_register
do not check this case. Some errors(malwares, etc.) may lead to calling
XEN_DOMCTL_bind_pt_irq without a clear gtable and will cause Xen panic.

Signed-off-by: Chao Gao <chao.gao@intel.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>