package/xen: security bump to version 4.12.2

package/xen: security bump to version 4.12.2

The 4.12.2 release brings a large number of fixes:

https://xenproject.org/downloads/xen-project-archives/xen-project-4-12-series/xen-project-4-12-2/

Including a number of security fixes:

XSA-296: VCPUOP_initialise DoS (CVE-2019-18420)
XSA-298: missing descriptor table limit checking in x86 PV emulation
	 (CVE-2019-18425)
XSA-299: Issues with restartable PV type change operations (CVE-2019-18421)
XSA-301: add-to-physmap can be abused to DoS Arm hosts (CVE-2019-18423)
XSA-302: passed through PCI devices may corrupt host memory after
         deassignment (CVE-2019-18424)
XSA-303: ARM: Interrupts are unconditionally unmasked in exception handlers
	 (CVE-2019-18422)
XSA-304: x86: Machine Check Error on Page Size Change DoS (CVE-2018-12207)
XSA-305: TSX Asynchronous Abort speculative side channel (CVE-2019-11135)
XSA-306: Device quarantine for alternate pci assignment methods
	 (CVE-2019-19579)
XSA-307: find_next_bit() issues (CVE-2019-19581 CVE-2019-19582)
XSA-308: VMX: VMentry failure with debug exceptions and blocked states
	 (CVE-2019-19583)
XSA-309: Linear pagetable use / entry miscounts (CVE-2019-19578)
XSA-310: Further issues with restartable PV type change operations
	 (CVE-2019-19580)
XSA-311: Bugs in dynamic height handling for AMD IOMMU pagetables
	 (CVE-2019-19577)

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>