qemu: store complete agent filesystem information

qemu: store complete agent filesystem information

In an effort to avoid holding both an agent and normal job at the same
time, we shouldn't access the vm definition from within qemu_agent.c
(i.e. while the agent job is being held). In preparation, we need to
store the full filesystem disk information in qemuAgentDiskInfo.  In a
following commit, we can pass this information back to the caller and
the caller can search the vm definition to match the filsystem disk to
an alias.

Signed-off-by: Jonathon Jongsma <jjongsma@redhat.com>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>