Xen PV backend: Move call to bdrv_new from blk_init to blk_connect

Xen PV backend: Move call to bdrv_new from blk_init to blk_connect

This commit delays the point at which bdrv_new (and hence blk_open
on the underlying device) is called from blk_init to blk_connect.
This ensures that in an inbound live migrate, the block device is
not opened until it has been closed at the other end. This is in
preparation for supporting devices with open/close consistency
without using O_DIRECT. This commit does NOT itself change O_DIRECT
semantics.

upstream-commit-id: 86f425db3b1c4b6c4a2927eaec35627f9ab2e703

Signed-off-by: Alex Bligh <alex@alex.org.uk>
Signed-off-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>