pc_sysfw: allow flash (-pflash) memory to be used with KVM

pc_sysfw: allow flash (-pflash) memory to be used with KVM

When pc-sysfw.rom_only == 0, flash memory will be
usable with kvm. In order to enable flash memory mode,
a pflash device must be created. (For example, by
using the -pflash command line parameter.)

Usage of a flash memory device with kvm requires
KVM_CAP_READONLY_MEM, and kvm will abort if
a flash device is used with an older kvm which does
not support this capability.

If a flash device is not used, then qemu/kvm will
operate in the original rom-mode.

Signed-off-by: Jordan Justen <jordan.l.justen@intel.com>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Message-id: 1369816047-16384-5-git-send-email-jordan.l.justen@intel.com
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>