Check and report for incomplete 'global' option format

Check and report for incomplete 'global' option format

Qemu might crash when provided incomplete '-global' option.
For example:
     qemu-system-x86_64 -global driver=isa-fdc
     qemu-system-x86_64: ../../devel/qemu/qapi/string-input-visitor.c:394:
     string_input_visitor_new: Assertion `str' failed.
     Aborted (core dumped)

Fixes: 3751d7c43f795b ("vl: allow full-blown QemuOpts syntax for -global")
Signed-off-by: Rohit Kumar <rohit.kumar3@nutanix.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Resolves: https://gitlab.com/qemu-project/qemu/-/issues/604
Message-Id: <20220216071508.412974-1-rohit.kumar3@nutanix.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>