OvmfPkg/build.sh: Enable flash for QEMU >= 1.6

OvmfPkg/build.sh: Enable flash for QEMU >= 1.6

If the QEMU version is found to be >= 1.6, then automatically
enable flash (using the QEMU pflash command line parameter).

QEMU supports flash since 1.2, but only if KVM is disabled.
As of QEMU 1.6, flash support should also be enabled when
KVM is used. Therefore it is safest to only enable flash for
QEMU 1.6 and newer.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Jordan Justen <jordan.l.justen@intel.com>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>
git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@14842 6f19259b-4bc3-4df7-8a09-765794883524