Introduce the X86_FW_OVMF Kconfig symbol for OVMF-specific code.
Move the OVMF-specific code from pc_sysfw.c to pc_sysfw_ovmf.c,
adding a pair of stubs.
Update MAINTAINERS to reach OVMF maintainers when these new
files are modified.

This fixes when building the microvm machine standalone:

  /usr/bin/ld: libqemu-i386-softmmu.fa.p/target_i386_monitor.c.o: in
  function `qmp_sev_inject_launch_secret':
  target/i386/monitor.c:749: undefined reference to `pc_system_ovmf_table_find'

Fixes: f522cef9 ("sev: update sev-inject-launch-secret to make gpa optional")
Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Acked-by: Michael S. Tsirkin <mst@redhat.com>
Message-Id: <20210616204328.2611406-22-philmd@redhat.com>