main/xen: upgrade to 4.15.0

From the release notices:

 The very old qemu-xen-traditional device model is deprecated, and no longer
 security supported, when used as a host process.

Drop musl-hvmloader-fix-stdint.patch as upstream contains multiple fixes for building on Alpine. Dito with the stdint_local.h and elf_local.h files (used by the dropped patch).

hotplug-vif-vtrill.patch rebased

gcc10-etherboot-enum.patch included in new upstream version of ipxe