qemu: Don't store disk alias in qemuAgentDiskInfo

qemu: Don't store disk alias in qemuAgentDiskInfo

The qemuAgentDiskInfo structure is filled with information received from
the agent command response, except for the 'alias' field, which is
retrieved from the vm definition. Limit this structure only to data that
was received from the agent message.

This is another intermediate step in moving the responsibility for
searching the vmdef from qemu_agent.c to qemu_driver.c so that we can
avoid holding an agent job and a normal job at the same time.

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