(2.13) kvm: Add migration capabilities as an hvparam

(2.13) kvm: Add migration capabilities as an hvparam

Latest QEMU versions support various migration capabilities.  Each
can be enabled/disabled with 'migrate_set_capability' monitor
command.

Version 1.7.0 defines x-rdma-pin-all, auto-converge, zero-blocks,
and xbzrle migration capabilities.

Since migration capabilities are more than one, and because hvparams
accept only strings/integers and not dicts we decide to provide the
capabilities that should be on via a ":" separated string.

In other words one can use the following to enable xbzrle and
auto-converge:

-H kvm:migration_caps=xbzrle:auto-converge

IMPORTANT: xbzrle may result to BSOD for instances running Windows
2008r8 on drbd.

Update man page of gnt-instance to include the new hvparam.

Signed-off-by: Dimitris Aragiorgis <dimara@grnet.gr>