block: Support GlusterFS as a QEMU block backend.

This patch adds gluster as the new block backend in QEMU. This gives
QEMU the ability to boot VM images from gluster volumes. Its already
possible to boot from VM images on gluster volumes using FUSE mount, but
this patchset provides the ability to boot VM images from gluster volumes
by by-passing the FUSE layer in gluster. This is made possible by
using libgfapi routines to perform IO on gluster volumes directly.

VM Image on gluster volume is specified like this:

file=gluster[+transport]://[server[:port]]/volname/image[?socket=...]

'gluster' is the protocol.

'transport' specifies the transport type used to connect to gluster
management daemon (glusterd). Valid transport types are
tcp, unix and rdma. If a transport type isn't specified, then tcp
type is assumed.

'server' specifies the server where the volume file specification for
the given volume resides. This can be either hostname, ipv4 address
or ipv6 address. ipv6 address needs to be within square brackets [ ].
If transport type is 'unix', then 'server' field should not be specifed.
The 'socket' field needs to be populated with the path to unix domain
socket.

'port' is the port number on which glusterd is listening. This is optional
and if not specified, QEMU will send 0 which will make gluster to use the
default port. If the transport type is unix, then 'port' should not be
specified.

'volname' is the name of the gluster volume which contains the VM image.

'image' is the path to the actual VM image that resides on gluster volume.

Examples:

file=gluster://1.2.3.4/testvol/a.img
file=gluster+tcp://1.2.3.4/testvol/a.img
file=gluster+tcp://1.2.3.4:24007/testvol/dir/a.img
file=gluster+tcp://[1:2:3:4:5:6:7:8]/testvol/dir/a.img
file=gluster+tcp://[1:2:3:4:5:6:7:8]:24007/testvol/dir/a.img
file=gluster+tcp://server.domain.com:24007/testvol/dir/a.img
file=gluster+unix:///testvol/dir/a.img?socket=/tmp/glusterd.socket
file=gluster+rdma://1.2.3.4:24007/testvol/a.img

Signed-off-by: Bharata B Rao <bharata@linux.vnet.ibm.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
