-global does not work for drivers that have a dot in their name, such as
cfi.pflash01.  This is just a parsing limitation, because such globals
can be declared easily inside a -readconfig file.

To allow this usage, support the full QemuOpts key/value syntax for -global
too, for example "-global driver=cfi.pflash01,property=secure,value=on".
The two formats do not conflict, because the key/value syntax does not have
a period before the first equal sign.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>