Add dump_image_format[] to qemu.conf and support compressed dump
at virsh dump. coredump compression is important for saving disk space
in an environment where multiple guests run.

In general, "disk space for dump" is specially allocated and will be
a dead space in the system. It's used only at emergency. So, it's better
to have both of save_image_format and dump_image_format. "save" is done
in scheduled manner with enough calculated disk space for it.

This code reuses some of save_image_format[] and supports the same format.

Changelog:
 - modified libvirtd_qemu.aug
 - modified test_libvirtd_qemu.aug
 - fixed error handling of qemudSaveCompressionTypeFromString()