xen/blkback: Move global/static variables into struct xen_blkbk.

Bundle the lot of discrete variables into a single structure.
This is based on what was done in the xen-netback driver:
xen: netback: Move global/static variables into struct xen_netbk.
(094944631cc5a9d6e623302c987f78117c0bf7ac)

Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>