qemu: virtiofs: kill the whole process group
qemu: virtiofs: kill the whole process group

Send TERM/KILL to virtiofsd and its child processes too
and do not exit until they are all dead.

https://bugzilla.redhat.com/show_bug.cgi?id=1940276

Signed-off-by: Ján Tomko <4cab11cfb98d3c937327354a78eb07dbb6ee2bc6@redhat.com>
Reviewed-by: Michal Privoznik <83d82aaba2eed257f4814b0c239c260c4caaadf0@redhat.com>
Tested-by: Masayoshi Mizuma <d5fddbc10d7d0c19347bebdeca1e9c9f92c33d10@jp.fujitsu.com>

nertpinx/libvirt,jfehlig/libvirt,crobinso/libvirt,olafhering/libvirt,nertpinx/libvirt,crobinso/libvirt,crobinso/libvirt,zippy2/libvirt,jfehlig/libvirt,libvirt/libvirt,zippy2/libvirt,libvirt/libvirt,nertpinx/libvirt,libvirt/libvirt,zippy2/libvirt,nertpinx/libvirt,olafhering/libvirt,nertpinx/libvirt,libvirt/libvirt,jfehlig/libvirt,olafhering/libvirt,olafhering/libvirt,crobinso/libvirt,jfehlig/libvirt,zippy2/libvirt