qemu: add vmcoreinfo support

qemu: add vmcoreinfo support

Starting from qemu 2.11, the `-device vmcoreinfo` will create a fw_cfg
entry for a guest to store dump details, necessary to process kernel
dump with KASLR enabled and providing additional kernel details.

In essence, it is similar to -fw_cfg name=etc/vmcoreinfo,file=X but in
this case it is not backed by a file, but collected by QEMU itself.

Since the device is a singleton and shouldn't use additional hardware
resources, it is presented as a <feature> element in the libvirt
domain XML.

The device is arm/x86 only for now (targets that support fw_cfg+dma).

Related to:
https://bugzilla.redhat.com/show_bug.cgi?id=1395248

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>