Enhance qemuParseCommandLineKeywords

Enhance qemuParseCommandLineKeywords

Current version expects name=value,... list and when an incorrect string
such as "a,b,c=d" would be parsed as "a,b,c" keyword with "d" value
without reporting any error, which is probably not the expected
behavior.

This patch adds an extra argument called allowEmptyValue, which if
non-zero will permit keywords with no value; "a,b=c,,d=" will be parsed
as follows:
    keyword value
    "a"     NULL
    "b"     "c"
    ""      NULL
    "d"     ""

In case allowEmptyValue is zero, the string is required to contain
name=value pairs only; retvalues is guaranteed to contain non-NULL
pointers. Now, "a,b,c=d" will result in an error.

Signed-off-by: Jiri Denemark <jdenemar@redhat.com>