Add "broadcast" option for mce command

When the following test case is injected with mce command, maybe user could not
get the expected result.
    DATA
               command cpu bank status             mcg_status  addr   misc
        (qemu) mce     1   1    0xbd00000000000000 0x05        0x1234 0x8c

    Expected Result
           panic type: "Fatal Machine check"

That is because each mce command can only inject the given cpu and could not
inject mce interrupt to other cpus. So user will get the following result:
    panic type: "Fatal machine check on current CPU"

"broadcast" option is used for injecting dummy data into other cpus. Injecting
mce with this option the expected result could be gotten.

Usage:
    Broadcast[on]
           command broadcast cpu bank status             mcg_status  addr   misc
    (qemu) mce     -b        1   1    0xbd00000000000000 0x05        0x1234 0x8c

    Broadcast[off]
           command cpu bank status             mcg_status  addr   misc
    (qemu) mce     1   1    0xbd00000000000000 0x05        0x1234 0x8c

Signed-off-by: Jin Dongming <jin.dongming@np.css.fujitsu.com>
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>
