OvmfPkg/build.sh: Parse qemu version in more cases

OvmfPkg/build.sh: Parse qemu version in more cases

Since qemu v2.7.0, the pkgversion appears to have a bug:

$ ./configure --target-list=x86_64-softmmu --with-pkgversion=foo

Results in this output:

$ x86_64-softmmu/qemu-system-x86_64 -version
QEMU emulator version 2.8.90(foo)
Copyright (c) 2003-2017 Fabrice Bellard and the QEMU Project developers

This appears to have been introduced in:

67a1de0d19 Makefile: Derive "PKGVERSION" from "git describe" by default

The previous commit (077de81a4c) produces this output:

$ x86_64-softmmu/qemu-system-x86_64 -version
QEMU emulator version 2.6.50 (foo), Copyright (c) 2003-2008 Fabrice Bellard

Now the OvmfPkg/build.sh script uses grep with '-o' to return only the
matched text.

grep -E is also used with a simple regex to extract only the digits of
the version.

qemu-bug: https://bugs.launchpad.net/bugs/1673373

Cc: Laszlo Ersek <lersek@redhat.com>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Jordan Justen <jordan.l.justen@intel.com>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>