libvirt: introduce libvirt/libvirt-common.h.in

libvirt: introduce libvirt/libvirt-common.h.in

As it turned out, we need to share some enums and declarations between
libvirt.h and libvirt-admin.h, but since our policy forbids direct includes of
libvirt*.h, there has to be some header exempt from this rule. This patch moves
the relevant part of code from libvirt.h.in to libvirt-common.h.in. Moreover,
since there is no need to have libvirt.h generated anymore, introduce a new
header libvirt.h which was previosly ignored from git and make the common
header ignored and generated instead.