main/xen: security fixes (CVE-2014-7154, CVE-2014-7155, CVE-2014-7156)

XSA-104	CVE-2014-7154 	Race condition in HVMOP_track_dirty_vram
XSA-105	CVE-2014-7155 	Missing privilege level checks in x86 HLT, LGDT,
                        LIDT, and LMSW emulation
XSA-106	CVE-2014-7156 	Missing privilege level checks in x86 emulation
                        of software interrupts

fixes #3460