hmp: Make "info block" output more readable

HMP is meant for humans and you should notice it.

This changes the output format to use a bit more space to display the
information more readable and leaves out irrelevant information (e.g.
mention only that an image is encrypted, but not when it's not; display
I/O limits only if throttling is in effect; ...)

Before:

    (qemu) info block
    ide0-hd0: removable=0 io-status=ok file=/tmp/overlay.qcow2
    backing_file=/tmp/backing.img backing_file_depth=1 ro=0 drv=qcow2
    encrypted=1 bps=0 bps_rd=0 bps_wr=0 iops=1024 iops_rd=0 iops_wr=0
    ide1-cd0: removable=1 locked=0 tray-open=0 io-status=ok
    file=/home/kwolf/images/iso/Fedora-18-x86_64-Live-Desktop.iso ro=1
    drv=raw encrypted=0 bps=0 bps_rd=0 bps_wr=0 iops=0 iops_rd=0 iops_wr=0
    floppy0: removable=1 locked=0 tray-open=0 [not inserted]
    sd0: removable=1 locked=0 tray-open=0 [not inserted]

After:

    (qemu) info block
    ide0-hd0: /tmp/overlay.qcow2 (qcow2, encrypted)
        Backing file:     /tmp/backing.img (chain depth: 1)
        I/O limits:       bps=0 bps_rd=0 bps_wr=0 iops=1024 iops_rd=0 iops_wr=0

    ide1-cd0: /home/kwolf/images/iso/Fedora-18-x86_64-Live-Desktop.iso (raw, read-only)
        Removable device: not locked, tray closed

    floppy0: [not inserted]
        Removable device: not locked, tray closed

    sd0: [not inserted]
        Removable device: not locked, tray closed

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Fam Zheng <famz@redhat.com>
Acked-by: Anthony Liguori <aliguori@us.ibm.com>
Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>
