target/i386: check for availability of MSR_IA32_UCODE_REV as an emulated MSR

target/i386: check for availability of MSR_IA32_UCODE_REV as an emulated MSR

Even though MSR_IA32_UCODE_REV has been available long before Linux 5.6,
which added it to the emulated MSR list, a bug caused the microcode
version to revert to 0x100000000 on INIT.  As a result, processors other
than the bootstrap processor would not see the host microcode revision;
some Windows version complain loudly about this and crash with a
fairly explicit MICROCODE REVISION MISMATCH error.

[If running 5.6 prereleases, the kernel fix "KVM: x86: do not reset
 microcode version on INIT or RESET" should also be applied.]

Reported-by: Alex Williamson <[email protected]>
Message-id: <20200211175516[email protected]>
Signed-off-by: Paolo Bonzini <[email protected]>