hmp: fix bad value conversion for M type

The M type converts from megabytes to bytes. However, the value can be
negative before the conversion, which will lead to a flawed conversion.

For example, this:

 (qemu) balloon -1000000000000011
 (qemu)

Just "works", but the value passed by the balloon command will be
something else.

This patch fixes this problem by requering a positive value before
converting. There's really no reason to accept a negative value for
the M type.

Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
