Restore request-based mode to xen-blkfront for AWS kernels

In current Ubuntu kernels, PV blkfront drivers have blk-mq enabled by default and cannot use the old I/O scheduler.

[Impact]
blk-mq is not as fast as the old request-based scheduler for some workloads on HDD disks.

[Fix]
Amazon Linux has a commit which reintroduces the request-based mode. It disables blk-mq by default but allows it to be switched back on with a kernel parameter.

For X this needs a small patch from upstream for error handling.

For B/C this patchset is bigger as it includes the suspend/resume patches already in X, and a new fixup. These are desirable as the request mode patch assumes their presence.

[Regression Potential]
Could potentially break xen based disks on AWS.

For B/C, the patches also add some code to the xen core around suspend and resume, this code is much smaller and also mirrors code already in Xenial.

[Tests]
Tested by AWS for Xenial, and their kernel engineers vetted the patches. I tested the Bionic and Cosmic patchsets with fio, the system appears stable and the IOPS promised for EBS Provisioned IOPS disks were met in my testing. I did an apt update/upgrade and everything worked (no hash-sum mismatches).