Monitor: handle optional '-' arg as a bool

Historically, user monitor arguments beginning with '-' (eg. '-f')
were passed as integers down to handlers.

I've maintained this behavior in the new monitor because we didn't
have a boolean type at the very beginning of QMP. Today we have it
and this behavior is causing trouble to QMP's argument checker.

This commit fixes the problem by doing the following changes:

1. User Monitor

   Before: the optional arg was represented as a QInt, we'd pass 1
           down to handlers if the user specified the argument or
           0 otherwise

   This commit: the optional arg is represented as a QBool, we pass
                true down to handlers if the user specified the
                argument, otherwise _nothing_ is passed

2. QMP

   Before: the client was required to pass the arg as QBool, but we'd
           convert it to QInt internally. If the argument wasn't passed,
           we'd pass 0 down

   This commit: still require a QBool, but doesn't do any conversion and
                doesn't pass any default value

3. Convert existing handlers (do_eject()/do_migrate()) to the new way

   Before: Both handlers would expect a QInt value, either 0 or 1

   This commit: Change the handlers to accept a QBool, they handle the
                following cases:

                   A) true is passed: the option is enabled
                   B) false is passed: the option is disabled
                   C) nothing is passed: option not specified, use
                                         default behavior

Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>
