qemuDomainAttachDiskGeneric: Split up frontend and backend attachment
qemuDomainAttachDiskGeneric: Split up frontend and backend attachment

Split up the monitor contexts to attach the backend of the disk and the
frontend device in preparation for hotplugging transient disks where
we'll need to add the code for adding the transient overlay between
these two steps.

Signed-off-by: Peter Krempa <2cf5c04c61aa466e4a47bfedc747d17279c72ffc@redhat.com>
Reviewed-by: Ján Tomko <4cab11cfb98d3c937327354a78eb07dbb6ee2bc6@redhat.com>
Reviewed-by: Pavel Hrdina <d4772d05997b8abf035041e3b4f4996380ea7e7a@redhat.com>

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